1. Field of the Invention
The present invention relates to a method for manufacturing electronic parts, which are exemplified by electronic parts made of so-called multilayer ceramic formed by laminating ceramic green sheets. More particularly, the present invention relates to a method for manufacturing what is called a ceramic green sheet incorporating an electrode layer inside. Examples of the multilayer ceramic electronic parts described herein include multilayer ceramic capacitors, multilayer ceramic inductors, LC composite parts including them and EMC related parts.
2. Related Background Art
In recent years, with downsizing and rapid popularization of electronic apparatuses represented by cellular phones, an increase in mounting density of the electronic parts used for those apparatuses and improvement in their performance, are required. To meet these requirements, demands for downsizing and improvement in functions, which will be realized by a decrease in the layer thickness and an increase in the number of layers, are placed on multilayer ceramic electronic parts that are used as passive elements. In addition, development of the manufacturing method that can meet these demands is also demanded.
In the following, a method for manufacturing a multilayer ceramic electronic mentioned above will be briefly described taking a multilayer ceramic capacitor having electrodes formed inside it as an example. In this technology, a plurality of electrodes are firstly formed on a ceramic green sheet made of a single ceramic substance at one time by means of a screen printing or the like using an electrically conductive paste containing a metal powder and an organic binder. Subsequently, a plurality of raw ceramic green sheets and ceramic green sheets on which electrodes have been formed etc. are stacked to form a ceramic multilayer member. The electrodes will constitute internal electrodes of multilayer ceramic electronic parts as finished products. Furthermore, the ceramic multilayer member is pressed in its thickness direction so that the green sheets will be brought into close contact with one another. The multilayer member having been brought into close contact is then cut into a certain size and separated into individual chips. External electrodes are suitably formed on the surface of the obtained chips or on the chips having been subjected to sintering. Thus, multilayer ceramic electronic parts are obtained.
There are many reports on methods for producing a ceramic green sheet in connection with multilayer ceramic electronic parts formed by laminating the ceramic green sheets. (See Japanese Patent Nos. 3216627, 3164068 and 3231987 and Japanese Patent Application Laid-Open NO. 2001-110662.)
Among them, a method in which through holes and patterned grooves are formed utilizing a photolithography technique including the process of applying a slurry, drying, exposure and development, and filling with a conductive material is receiving attention. This is because in view of the fact that various electrode patterns formed in a sheet have become finer with downsizing of the multilayer ceramic electronic parts, the photolithography technique is introduced as a method suitable for fine processing.
More specifically, a typical process including the process of applying a photosensitive slurry, forming thereafter a desired electrode pattern by photolithography and filling the negative pattern thus formed with an electrode material.
When the photolithography technique is used, what is important is resolution and light transmissivity.
Resolution has an influence on the precision of the shape of the pattern formed. In the case of a ceramic green sheet having high light diffracting characteristics such as a glass ceramic green sheet, resolution is associated with the problems such as lack of sharpness at the edge of the outer shape of the pattern caused by an influence of diffracted light. Especially, the farther from the exposed side along the sheet thickness direction, the more the influence matters. In other words, there is the problem that portions that need not to be exposed are also exposed due to diffraction caused by the member to be exposed.
Light transmissivity has an influence on the cross sectional shape of the pattern formed. In the case of a ceramic green sheet having a low light transmissivity such as a ferrite ceramic green sheet, light transmissivity is associated with problems such as tapering of the pattern cross section in accordance with a decrease in the light quantity. In this case also, the farther from the exposed side along the sheet thickness direction, the more the influence matters. In other words, there is the problem that portions that are required to be exposed are not exposed due to opacity of the member to be exposed.
In many cases, the aforementioned exposed pattern is filled with an electrically conductive material, and it is desirable that the cross sectional shape of the pattern be rectangular as far as possible, in view of, for example: 1) ease of filling with the electrically conductive paste and 2) reduction of the DC resistance of the electrodes.
In view of the above-described circumstances, when a thick sheet incorporating an electrode pattern etc. is to be formed through the conventional process, a plurality of layers each of which has a thickness that does not affect the resolution and the light transmissivity should be stacked separately or the process including slurry application, exposure, development and electrode printing should be repeated plural times. For example, in the case that a layer with a thickness of 20 μm is to be produced using a material that can transmit or is sensitive to light only up to a thickness of 2 μm, it is necessary with the conventional process to form and stack ten layers. Such a stacking or repeating process invites deterioration of stacking accuracy and is disadvantageous in terms of reliability in electrical conduction characteristics.
In view of the above, a method for producing a ceramic green sheet that is improved in stacking accuracy and electrical conduction reliability while not affected by the resolution and light transmissivity is desired.
As to resolution, solutions such as smoothing the surface of insulating layers and mixing colorant for absorbing reflected light are disclosed in Japanese Patent No. 3216627 and Japanese Patent Application Laid-Open No. 5-271576 (which is cited in Japanese Patent No. 3216627). However, there is no prior art that offers a solution for the problem concerning light transmissivity.
For the problems concerning diffraction in a photosensitive thick film, contamination of a mask in contact exposure and resolution in proximity exposure, Japanese Patent No. 3216627 discloses as a solution a method for manufacturing an inductor. In this method, a photosensitive, electrically conductive thick film is formed, then it is subjected to smoothing and a pattern is formed therein by exposure using a mask and development, a photosensitive insulating thick film is formed thereon, then it is subjected to smoothing and a through holes are formed by exposure and development, a photosensitive, electrically conductive thick film is formed thereon to fill the through holes, then it is subjected to smoothing and a pattern is formed by exposure and development. This method utilizes the method of forming a photosensitive insulating film on a conductive material pattern, then exposing and developing it. However, the photosensitive insulating film on the conductive material pattern is formed as a thick layer and through holes are formed by exposure and development. Accordingly, there is a demand for a simpler process having wider applicability.
The technology disclosed in Japanese Patent No. 3164068 has as an object to increase the thickness of the conductive material pattern. In the method disclosed in this document, an electronic part is manufactured by repeatedly performing the process of forming a photosensitive electrically conductive thick film, subsequently forming a pattern therein by exposure using a mask and development, forming a film of an insulating material thereon, then removing the insulating layer using a solvent until the upper surface of the underlying lower conductor pattern is exposed, forming an upper conductive pattern layer on the lower conductive pattern, forming a photosensitive insulating thick film thereon, then forming via holes by exposure and development, forming a photosensitive, electrically conductive layer, and filing the via holes by exposure using a mask and development. However, according to this method, the film thickness is decreased since portions of the insulating film other than the portion on the conductor pattern are also dissolved, and therefore there is the problem of stack misalignment.
Japanese Patent No. 3231987 discloses a method for manufacturing a ceramic circuit board in which a laminated member is formed by repeatedly performing the process of applying a photosensitive ceramic slip material (considered to refer to slurry), forming a pattern groove for internal wiring by exposure and development and thereafter filling the groove with conductive paste, and the laminated member thus formed is sintered. This method is developed from the printing-lamination method, and it forms a laminated member in a building-up manner. In other words, in forming laminated layers successively, formation of one layer is not completed unless all of the sequential process steps of application, drying, exposure, development, liquid removal or drying, filling with conductor, drying, filling with resin and drying (or exposure). Therefore, according to this prior art, if a defective layer is generated in the course of the process, all the layers that have been formed up to then may sometimes be wasted, and it is disadvantageous in lead time and yield.
Japanese Patent Application Laid-Open No. 2001-110662discloses a method for manufacturing a part incorporating an inductor in which a photosensitive slurry is applied on an electrically conductive processing material, a pattern groove is formed thereon by exposure and development, electrodes are formed in the groove by electrodeposition, layers formed in this way are stacked and then sintered. However, this method suffers from the problems that the thickness (or the resolution or diffraction characteristics) of the insulating layer affects the cross sectional shape of the internal electrode pattern and that the number of the process step increases. Furthermore, in this method there are restrictions due to resolution dependency in resolving at high aspect and production of thick layers in the case of the application of the photosensitive slurry and the electrodeposition of the photosensitive powder.